Systems for storing or transporting fluids that may damage the environment, such as caustic or volatile fluids, are often double-wall systems. These systems include a primary container that contains the fluid and a secondary container that contains the primary container. As a result, if the primary container leaks, the fluid is contained within the secondary container.
After such systems are installed, they are typically tested, which may be required by governmental regulations, to make sure that the primary container is functioning properly, i.e., not leaking. Sometimes these systems are buried underground. For example, petroleum dispensers are typically connected to underground storage tanks that contain the petroleum. The underground tanks as well as the pipe connecting the tanks to the dispensers are almost always primary containers that are surrounded and sealed within secondary containers. Even though the primary container is intended for liquids, the integrity of the system is typically determined by testing for vapor leaks.
Testing these, and other underground systems, after installation poses challenges because most of the containment system is buried and therefore not accessible except for portions that communicate with the surface, such as manways, risers and the like. Another challenge, regardless of the system location, is that the primary container may be made of a polymeric material that is somewhat permeable to any vapors it contains. In other words, there is a continuous, albeit small, transmission of vapor within the primary container into the interstitial space between the primary and secondary containers. This may need to be taken into account to accurately determine whether there is a vapor leak over and above that expected as a result of permeation.
Furthermore, when a primary system is first pressurized for testing, typically with a tracer gas, very little fluid permeates into the interstice between the primary and secondary containers. But permeation increases over time. When a primary vessel under test has been pressurized for a period before the test begins, concentrations of interstitial tracer gas are higher at the start of the test than if the vessel was pressurized immediately prior to the test start. These higher concentrations that result from permeation should be considered when determining the integrity of the primary vessel. Aside from the permeation issues, when a primary vessel is a pipe, especially a buried one, it would be desirable to approximate the location of any leak detected so that only a relatively small portion of the pipe system needed to be exposed by digging and then repaired.